It is known that an internal resistance of a fuel cell influences wettability of an electrolytic film in the fuel cell. When the fuel cell has a small internal water content and the electrolytic film dries (so-called dry-up), the internal resistance increases, and an output voltage of the fuel cell drops. On the other hand, when the fuel cell includes an excessively large internal water content (so-called flooding), electrodes of the fuel cell are covered with the water content, hence dispersion of oxygen or hydrogen as a reactant is disturbed, and the output voltage drops.
To efficiently operate the fuel cell, optimum control of the internal water content of the fuel cell needs to be performed. The internal water content of the fuel cell has correlation with an impedance of the fuel cell. At present, the impedance of the fuel cell is measured by an alternating-current impedance process, and a water content state in the fuel cell is indirectly grasped.
For example, in the following patent document 1, a method is disclosed in which a sinusoidal signal (a signal for impedance measurement) having an arbitrary frequency is applied to (superimposed on) an output signal of the fuel cell, and the impedance in this case is measured to grasp the water content state in the fuel cell in real time.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-86220